Control System design of a H-shape configuration Fixed Wing VTOL based on a Patented Vertical Thrust System

Urban air mobility (UAM) can become an integral part of worldwide urban mobility for last-mile connections (up to 50 km range) or sub-regional routes (from 100 to 300 km range). To work in future urban scenarios, environmentally friendly air vehicles are designed to be able to perform vertical take-off and landing (VTOL). Although many challenges are expected to be faced for enabling technologies, infrastructures and air space management, many prototypes of UAM vehicles are proposed. VTOL characteristics can be easily achieved by a multirotor configuration in order to produce a vertical force to balance the weight of the system. However, such a configuration limits the aerodynamic efficiency and poses noticeable endurance restrictions, thus limiting the operability of the UAVs to local missions only. A first approach to improve the flight efficiency is to design a hybrid configuration with both Fixed Wing (FW) and MultiRotor (MR) capabilities in order to exploit lift generation in horizontal flight. The hybrid nature of the vehicle implies several design complications in terms of transition phase from MR to FW mode during which the system is extremely over-actuated. Moreover, for multiple people carrying applications, the vertical thrust required for vertical operations involves large rotors. This could significantly penalize the aerodynamic performance of the aircraft in FW configuration. In order to overcome the aforementioned limitations, a novel solution based on the patented system ThrustPod is proposed. A conventional FW aircraft is equipped with rotors that are activated only during vertical flight phases. On the contrary, during the FW cruise phase, the rotors are retracted inside the fuselage, and the horizontal thrust is provided by dedicated tail propellers. The typical mission of the vehicle would include MR vertical take-off and landing and FW cruising with a transition to mildly transfer the control functions from the vertical rotors to the aerodynamic surfaces. The transition regime causes additional difficulties in the control system design as control inputs from MR and FW configurations are simultaneously active. Therefore, a control allocation scheme is needed to adequately distribute the control load on both MR and FW control variable sets for the best exploitation of the aerodynamic forces. The proposed idea is to gradually prioritize the aerodynamic control surfaces over MR actuators as the speed of the vehicle is increased. The vehicle is represented as a state machine in which each phase corresponds to a state. First MR state is activated to perform vertical flight and hovering. As horizontal speed increases, the second state is initiated and a control system for the transition phase comes into play. A control load placement over the multiple actuators is investigated in order to provide a controlled altitude transition. The control inputs are weighted with a continuous function of velocity and an Incremental Nonlinear Dynamic Inversion (INDI) technique is adopted. At this stage an INDI approach is preferred over the classical NDI as it is less reliant on the mathematical model of the plant, consequently it allows to use a less accurate aerodynamic model. The third state of the state machine (i.e. Fixed Wing flight) is activated at around stall velocity. However the same control logic as the transition phase can be retained, with exception of providing all the required forces and torques through aerodynamic surfaces only.